Tenth leaf or leaflet petiole exudate colour and perception

These features refer to the tenth leaf stage. The tenth leaf is the tenth true leaf produced after the cotyledons. On seedlings with alternate or spirally arranged leaves it is just simply the tenth leaf produced.  On seedlings with opposite leaves it is one of the leaves in the fifth pair of true leaves produced.  On seedlings with whorled leaves it will be one of the leaves in the appropriate whorl; e.g. if there are four leaves in each whorl then the tenth leaf will be one of the leaves in the third whorl.

Freshly cut or broken petioles from living seedlings of certain species will produce exudates